ACFT EQUIP DESIGN PROB PREVENTS DETECTION OF ACFT EQUIP MALFUNCTION.

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AFTER TAXI TO THE GATE AT COMPLETION OF FLT; WE WERE MADE AWARE OF A LNDG GEAR DOOR TOUCHING THE GND BY RAMP PERSONNEL INSPECTION SHOWED R MAIN FUSELAGE MOUNTED DOOR TOUCHING GND; AND APPROX 12 INCHES MISSING. APCH AND TOUCHDOWN WERE NORMAL. FINAL LNDG CHKS WERE COMPLETED AT 500-700 FT AGL. AT THAT TIME NEITHER I NOR THE CAPT OBSERVED A 'GEAR DOOR OPEN' LIGHT ON AND WE BOTH FEEL THAT WE WOULD HAVE SEEN IT IF IT WERE ON. AFTER SHUTDOWN AT THE GATE; THE 'GEAR DOOR OPEN' LIGHT WAS SEEN TO BE ILLUMINATED; WHICH WOULD BE NORMAL. ONE FLT ATTENDANT DID RPT HEARING A NOISE; WHICH SHE THOUGHT WAS 'A TIRE OR SOMETHING.' OP WAS AT A RELATIVELY QUIET TIME AND TWR CTLR WAS ALSO FUNCTIONING AS GND CTLR. WE WERE NEVER ALERTED TO A GEAR DOOR BEING DOWN OR DRAGGING THE GND DURING TAXI. A CHK ON THE RWY BY ARPT PERSONNEL SHOWED NO STRIKE MARKS OR PIECES OF THE GEAR DOOR UNTIL RIGHT AS WE TURNED OFF OF THE RWY ONTO TXWY A4. THE CAPT WAS BUSY TAXIING THE ACFT TO THE GATE AND I WAS BUSY CLEANING UP THE ACFT AND TALKING TO COMPANY. SO IF THE 'GEAR DOOR OPEN' LIGHT WAS ON; NEITHER OF US OBSERVED IT. I FEEL THERE ARE 2 MORE CONTRIBUTING FACTORS. THE 'GEAR DOOR OPEN' LIGHT IS NOT WIRED TO THE MASTER CAUTION LIGHT; AND THE LIGHT POS MAKES IT OUT OF DIRECT VIEW DURING VISUAL - - TYPE APCHS. A RECENT MODIFICATION TO MOUNT WIND SHEAR WARNING AND CAUTION LIGHTS UNDER THE EDGE OF THE GLARE SHIELD EFFECTIVELY BLOCKS THE 'GEAR DOOR OPEN' LIGHT FROM VIEW IF SEAT IS ADJUSTED NORMALLY. I AM FIRMLY CONVINCED THAT THE 'GEAR DOOR OPEN' LIGHT WAS NOT ILLUMINATED DURING OUR LNDG CHKS. IF IT DID COME ON LATER; IT WAS NOT READILY VIEWABLE FROM MY SEAT POS. MAINT DID FIND A BROKEN BOLT ON A DRAG LINK BTWN THE MAIN GEAR AND A DOOR SEQUENCING BELL CRANK. THE BROKEN BOLT ALLOWED THE DOOR TO OPEN UNCOMMANDED.